Empty set is a set with no element (the number of elements in the empty set is zero).
A set is subset set of another set B if it its elements belong also to subset B.
Because an empty set has no element its elements (there are none) belong to any set B. So the empty set is a subset of any set.
Example: Set A:The set of all people who can fly Set B:The set of all people.
There are no elements in the set A (no one has wings to fly), so set A is a subset of Set B.